The present invention relates to wireless networks, and in particular to a method and system for locating a transmitting radio tag in an infrastructure wireless local area network (WLAN), for locating a client station in the WLAN, and for locating a coverage hole in the WLAN.
It has long been known to use wireless technology to track transmitting radio transmitters. One application of such wireless locating methods is to locate active transmitting devices that are attached to assets. Such transmitting devices are called radio tags, or simply tags herein. The tags often are arranged to transmit a signal optimized for location accuracy. Known tracking methods for such radio tags typically rely on estimating the range from the tag to a plurality of receiving stations at known reference locations. Techniques for performing the range estimation include methods based on estimating the propagation delay, i.e., a time domain technique, or methods based on estimating the path loss from a tag to be tracked to a plurality of receiving stations at known locations.
It has long been known to use wireless technology for packet networks. Packet networks that use wireless links are known as wireless networks. For example, wireless local area networks (wireless LANS, WLANS) based on the IEEE 802.11 standard, commonly known as WiFi, are now becoming ubiquitous.
The present invention relates to using wireless stations of a wireless network for wireless tracking. Furthermore, the present invention relates to such wireless tracking of two types of devices:                1. Radio tags, used herein to mean radio devices that can transmit, and whose main purpose is to be tracked and identified.        2. WLAN clients, used herein to mean devices whose primary purpose is to communicate in an infrastructure WLAN via an access point. The communication can be of transfer voice, video, or data. While the primary purpose of a WLAN client is communication of messages, such devices can also be tracked.        
There are many applications for tracking radio tags. Typically, most would involve attaching a radio tag to a high-value asset, e.g., a large component on an assembly line, a medical instrument, or even to children in a large public space.
In one exemplary application, the radio tag is pre-programmed to transmit on a pre-defined channel set, at a pre-specified transmit power, repeating transmission at a pre-defined time interval, and for a pre-defined duration, including, assuming battery operation, until the battery power runs down.
Desirable features of such radio tags include: small size, long lifetime, and long range. There therefore is a desire to make the transmission extremely efficient, e.g., by only transmitting for a relatively short time.
There also are many applications for tracking WLAN clients. Such applications include:                Security: Estimating the location of a wireless client before a decision is made whether or not to authenticate the client.        Safety: Location services such as E911 whereby a person in distress can use a wireless client station to send a message indicating assistance is needed. Location technology would enable emergency personnel to locate the client station quickly.        Network Management and Troubleshooting: A client that is experiencing poor wireless LAN performance requests to be tracked by a management entity, and the location information can help the network operator make a decision that there is either something wrong with the network, or something wrong with the client device. The operator could also use locating tracking capability to examine when and where the network gets overloaded with voice or data devices.        
One desirable feature of radio tags is infrequent recharging or battery replacement. Thus, a typical tag may require recharging or battery replacement once every few years. Typical wireless LAN clients get recharged once every few hours. Therefore, in the case of client locating, power efficiency is not as much of a concern as with radio tags. Flexibility is much more important. It is desirable for wireless clients to have their channel sets, transmit power, transmit interval, and transmit durations adjusted as the need arises. It is also desirable to enable existing “legacy” clients to be easily upgraded for location capability, e.g., using software modifications only rather than having to replace hardware components.
There is thus a need in the art for a unified location method and system that includes one or more of these desirable aspects, e.g., that can use substantially standard WLAN infrastructure, and thus that can be widely deployed quickly.
There is also a need in the art for a unified location method and system that provides for interoperability among vendors. For example, there is a need in the art for a single location method and system that can locate tags and client devices manufactured by multiple different companies.
A number of techniques have been proposed for radiolocation. Methods are known, for example, that rely on the Global Positioning System (GPS). GPS, however, is known to have poor indoor reception and long acquisition time. GPS also requires additional GPS hardware in the wireless station that would increase the cost of stations, e.g., client devices.
Methods also are known for radiolocation that rely on time difference of arrival (TDOA) estimation. Such methods require relatively precise time synchronization at each station.
Methods also are known for radiolocation in a WLAN that use signal strength measurements, e.g., RSSI measurements using existing mobile client station hardware. Some such methods use training to form a map of propagation characteristics that in turn requires taking time-consuming signal strength measurements at numerous locations by a cooperative mobile client station. Examples of methods that use training include that described in Small, J., Smailagic, A., and Siewiorek, D. P. (2000): “Determining User Location For Context Aware Computing Through the Use of a Wireless LAN Infrastructure,” Pittsburgh, USA: Institute for Complex Engineered Systems. Carnegie-Mellon University, Pittsburgh, Pa. 15213, 2000. Available at http://www-2.cs.cmu.edu/˜aura/docdir/small00.pdf. Another such method is that described in P. Bahl and V. N. Padmanabhan, RADAR: “An In-Building RF-based User Location and Tracking System,” IEEE Infocom 2000, vol. 2, Mar. 2000, pp. 775-784. Yet another is described in Kishan, A., Michael, M., Rihan, S., and R. Biswas: “Halibut: An Infrastructure for Wireless LAN Location-Based Services,” Technical paper for Course CS444n, Computer Science Department, Stanford University, Stanford Calif., June 2001. Previously available at http://fern2.stanford.edu/cs444n/paper.pdf.
A prior art method also is known for WLANs that uses RF modeling. The modeling, however, requires detailed input of building layout, wall location, and construction materials. Some of the papers and technical reports mentioned in the previous paragraph include such models.
U.S. Pat. No. 6,664,925 issued Dec. 16, 2003 to Moore et al. and titled APPARATUS AND METHOD FOR MAPPING A LOCATION OF WIRELESS BASE STATIONS IN A MOBILE COMMUNICATION SYSTEM uses RSSI measured at client stations from APs at known regions to determine which of the regions the client station is at, e.g., which AP location is the closest. A variation is mentioned which uses the inverse square propagation model to also attempt to describe how close to the nearest AP the client may be. The Moore et al. method provides relatively coarse measures and requires the location of the APs to be known in advance. Furthermore, the Moore et al. method is described for client stations only and not as a unified method suitable for tag location, WLAN coverage area gap determining, and client station location. Furthermore, the Moore et al. method does not include any field calibration to correct AP measurements by using AP to AP path loss measurement.
Above referenced Kaiser et al. invention (U.S. patent application Ser. No. 10/629,384, incorporated herein by reference) described a method of locating an unknown client station or a rogue client station. One embodiment of the Kaiser et al. invention operates in a managed wireless network in which the APs and their clients are managed by a central management entity. The managed wireless network substantially conforms to the IEEE 802.11 standard in that the network is compatible with that standard, and includes slight modifications, such as additional MAC frames that are used to convey information such as transmit power. Furthermore, stations of the network measure the received signal strength relatively accurately. By a managed access point is meant an access points at a known location whose transmit power is known and whose received signal strength is measurable.
An implementation of the Kaiser et al. method includes accepting an ideal path loss model and calibrating the ideal path loss model using path loss measurements between a first set and a second set of managed access points of a managed wireless network in an area of interest. The stations of the first and second sets are at known locations. In one embodiment, the first and second sets are the same set of managed access points of the managed wireless network located in the area of interest. The path loss measurements are obtained using measurements received from the first set that measure the received signal strengths at each of the respective APs of the first set as a result of transmissions by each wireless station of the second set. Each transmission by a respective station of the second set is at a known respective transmit power. The method further includes measuring the path loss between the wireless station of an unknown location and at least some of the managed access points. The calibrating determines a calibrated path loss model between the access points. Such a calibrated model is then used to locate a client station. The client may be a managed client of one of the managed access points transmitting at a transmit power whose value can be determined at the receiving AP, or a rogue station transmitting at an unknown power.
Known methods of radiolocation using signal strength measurements may be unsuitable for tag location because of the requirements for tags to have long battery life and automated operation.
There thus still is a need in the art for a unified method to track radio tags and client stations, and to determine gaps in coverage area. There further is a need in the art for a unified tag location, client location, and coverage gap location method that do not need a map or require field calibration measurements in order to provide a location estimate. There further is a need for a method that does not require but can use AP-to-AP field calibration measurements to improve performance.